1. Field of the Invention
The present invention relates to an adjustment method of a wireless transmission device, especially to a transmission rate adjustment method of a wireless transmission device.
2. Description of Related Art
Wireless transmission devices have been popular these years due to the convenience. Among the wireless transmission devices of different standards, the wireless transmission device in compliance with IEEE 802.11 standard has become the mainstream in short distance transmission applications. As the development of 802.11 standard (from 802.11, 802.11a, 802.11b, . . . , to 802.11n), the wireless transmission device nowadays is faster and supports longer transmission distance in comparison with the outdated products. Taking 802.11n for example, it supports the multiple input multiple output (MIMO) technique, supports multiple transmission rates, and is capable of reaching a transmission rate of 300 Mbps or more (e.g. 600 Mbps). However, not all the environments or equipments can sustain such high transmission rates. In a proper condition, a wireless transmission device complying with 802.11 standard can speed up to raise the transmission rate; however, in an improper condition, the wireless transmission device has to speed down to make sure that the data can be sent successfully.
A traditional rate adjustment method is carried out in accordance with the transmission status of a wireless transmission channel. The transmission status, for example, could be a packet retransmission count, a received signal strength indicator (RSSI) or a signal to noise ratio (SNR). Because traditional 802.11a/b/g standards do not support the MIMO technique but support the transmission of single spatial stream (SS), to a 802.11a/b/g wireless transmission device or a 802.11n wireless transmission device only using the transmission of single spatial stream, the rate adjustment of the current art is executed merely by the speed order of transmission rates or the order of modulation and coding schemes (MCSs). However, to a wireless transmission device using two or more spatial streams for transmission, the rate adjustment of the current art will no longer be enough. For instance, if a system composed of a wireless transmission device and a reception end is capable of supporting two independent spatial streams, the modulation and coding schemes which the system can support are MCS0 to MCS15 in which MCS0 to MCS7 belong to the schemes of a single spatial stream mode while MCS8 to MCS15 belongs to the schemes of a double spatial stream (DS) mode. Since the order of MCS0 to MCS15 is not completely arranged by their corresponding transmission rates, the aforementioned rate adjustment of the current art will suffer at least the following problems:
First problem: If the optimal transmission rate is MCS11 of the double spatial stream mode (corresponding to the rate of 108.0 Mbps) under the present environment and equipment, assuming that the current transmission rate is MCS0 of the single spatial stream mode (corresponding to the rate of 13.5 Mbps), then in the speed-up process according to the MCS order (i.e. MCS0, MCS1, MCS2 . . . ), MCS4 instead of MCS11 will be the speed-up result because when the transmission rate is sequentially raised to MCS5 of the single spatial stream mode (corresponding to the rate of 108.0 Mbps equal to that of the optimal choice MCS11), the transmission rate will fail due to the higher SNR requirement of the single spatial stream mode. As a result, the scheme MCS4 prior to the unsuccessful scheme MCS5 will be the final result of speed-up, and the optimal rate MCS11 will not be reached.Second problem: If the environment and equipment on hand only support the transmission under the single spatial stream mode, assuming that the current transmission rate is MCS15 of the double spatial stream mode, then in the speed-down process according to the MCS order (i.e. MCS15, MCS14, MCS13, . . . ), the precedential test of MCS15 to MCS8 will fail because these schemes all belong to the double spatial stream mode. As a result, the test will not be successful until the scheme MCS7 of the single spatial stream mode is adopted, which unavoidably consumes too much time and causes the throughput reduced.
In additional to the above-mentioned problems, the current art will only choose one transmission rate (i.e. a modulation and coding scheme) for test in order to avoid the interference to the data transmission under a normal mode, which consequently stalls off the time for finding the best transmission rate. Besides, the current art can not distinguish the transmission failure due to SNR from the transmission failure due to packet collision, and thus in the collision case the current art may lower the transmission speed unnecessarily, which causes the packet length extended and leads to more serious collision problems. In other words, if the cause of failure is uncertain, the speed-down process can not be done rapidly and properly. Moreover, before speed-up or speed-down, the current art doesn't have an appropriate algorithm in determining an initial transmission rate to accelerate finding the optimal transmission rate.